A typical dispatch two-way radio communication system comprises communication units, communication resources, communication sites, and a communication resource allocator. Each of the sites have a substantially distinct coverage area and are geographically located throughout the system. Each site also has a number of communication resources assigned to it, where at least one of the communication resources is used as a control channel, while a number of the remaining communication resources are used as voice channels. Such systems are known to use both frequency division multiple access (FDMA) and time division multiple access (TDMA) methods to rebroadcast transmissions.
In a system that supports dispatch group calls (one-to-many calls), the communication units are typically arranged into communication groups (talk groups) and may be located anywhere within the system (in any site). When a communication unit of a talk group requests a group call, it transmits, via a control channel of the site in which it is located, an inbound signaling message to the communication resource allocator. (A group call typically allows all members of the same talk group that are located within the system to communicate with each other.) The inbound signaling message generally comprises the requesting communication unit's individual identification number, identification of the requesting communication unit's talk group, and a request for a group call. Upon receiving the inbound signaling message, the communication resource allocator will allocate a voice channel (i.e., an FDM or TDM channel) in any necessary sites to the requesting communication unit's talk group. In this manner, rapid access communications (typically at the push of a single button) are provided. Dispatch-capable systems, frequently referred to as private systems, are often used by public safety and utility organizations given their need for group communications and rapid system access.
In the last decade, in response to an ever-accelerating worldwide demand for mobile and personal portable communications, spread spectrum digital technology, of which one type is known as code division multiple access (CDMA), has proved to be an alternative for serving large populations of multiple access users over analog or other digital technologies. CDMA relies on processing power to extract a coded signal embedded across a broad frequency spectrum. The only way to extract the wanted signal from among many other overlaid unwanted signals is to have the right code. The use of coding allows more channels to be derived by the overlaying of carriers one over another and greatly enhances performance in terms of derived channels per hertz of bandwidth. The rapid expansion of such cellular communication systems, often referred to as public systems, has decreased the cost for individual users to engage in wireless communications.
Often, users of private, dispatch communication systems are also users of public, cellular communication systems. As a result, providers of wireless communication services have recognized the need for systems which provide the cost efficiency of public systems and also provide the "mission critical" functionality commonly found in private systems. For example, an "IDEN" communication systems by Motorola, Inc. provides both one-to-one and one-to-many communication services based a TDMA architecture. In such a system, however, it is often difficult to dedicate one or more communication resources to a given service because of the constantly evolving system loading characteristics. That is, assuming a given load distribution between the one-to-one and one-to-many services, a number of communication resources can be dedicated to each type of service. However, if the actual load distribution varies from the assumed load distribution, the efficiency for one of the services (the one requiring more resources than it is assigned) decreases. As a result, time division multiplexed (TDM) and frequency division multiplexed (FDM) systems typically do not provide dedicated resources.
Thus, there exists a need for techniques and apparatus allowing a wireless system to provide both the cost efficiency of a public system and the functionality often found in a private system.